This non-provisional application claims priority under 35 U.S.C. xc2xa7119(a) on Patent Application No. 2001-107478 filed in Japan on Apr. 5, 2001, which is herein incorporated by reference.
1. Field of the Invention
This invention relates to an accumulator fuel injection system, and more particularly to an accumulator fuel injection technique for quickly activating an exhaust emission control device in a diesel engine.
2. Description of Related Art
Exhaust gases emitted from a diesel engine installed in a bus, truck, or the like include a lot of particulate matters as well as HC, CO, NOx, etc. Accordingly, a diesel particulate filter that takes hold of particulate matters and removes them by burning with an external heat source, and an oxidization catalyst that processes HC and CO have been developed as after treatment devices for diesel engines. Recently, a continuous regenerative diesel particulate filter has also been proposed in which instead of an external heat source that burns particulate matters deposited on a diesel particulate filter, a catalyst for generating NO2 serving as a device for supplying an oxidizing agent for oxidizing and removing particulate matters is provided upstream the diesel particulate filter to continuously process the particulate matters deposited on the diesel particulate filter by device of the generated NO2. Further, it has been proposed that an NOx catalyst with a composition intended for mainly processing NOx is provided in an exhaust passage.
It is known that such oxidizing agent, continuous regenerative diesel particulate filter, and NOx catalyst is able to satisfactorily function only in an atmosphere in which exhaust gases are activated at a relatively high temperature. Therefore, in the case where an engine is cold at the start or the like, the oxidizing catalyst, continuous regenerative diesel particulate filter, and NOx catalyst are required to be quickly activated and constantly maintained in an activated state.
Accordingly, a variety of techniques have been proposed in which a heat source such as an electric heater is provided in the oxidizing catalyst, continuous regenerative diesel particulate filter, and the NOx catalyst in order to quickly activate them by heating with the heat source at the start of the engine.
However, it is not preferable to additionally provide the heat source as mentioned above since it complicates the arrangement of the after treatment system and increases the cost.
On the other hand, in recent years, to control fuel injection for a diesel engine, a comnion rail system has been developed which is capable of injecting high-pressure fuel stored in an accumulator into a combustion chamber by electrically controlling the opening and closing of a fuel injection nozzle. A diesel engine utilizing the common rail system has such a characteristic that the system is capable of freely setting the fuel injection timing by changing the opening timing of a fuel injection nozzle. Namely, the use of the common rail system enables fuel injection not only in proximity to a top dead center of a compression stroke but also in all of an intake stroke, expansion stroke, and exhaust stroke.
Further, to prevent an increase in engine operating noises and NOx resulting from a rapid explosive combustion at the initial stage of combustion, a technique has been developed in which a small amount of fuel is injected at a low pressure at the initial stage of a fuel injection cycle (initial injection), and fuel is then injected at a high pressure. This technique has been applied to the above-mentioned common rail system.
Accordingly, by utilizing the characteristics of the common rail system, a technique has been developed in which the temperatures of the oxidizing catalyst, continuous regenerative diesel particulate filter, and NOx catalyst are raised by injecting additional fuel during and after an expansion stroke (post injection) after fuel is injected for main combustion, and then causing the additional fuel to be burned in a combustion chamber by flame or to be reacted on a catalyst in an exhaust passage to raise the temperature of exhaust.
However, if high-pressure fuel is continuously injected during the post injection after fuel is injected at a high pressure, the fuel is adhered to the wall surfaces of cylinder liners to cause oil dilution, seizure, etc. due to the strong penetration power of the injected fuel.
It is therefore an object of the present invention to provide an accumulator fuel injection system that is capable of preventing fuel from adhering to the surface walls of cylinder liners by keeping a fuel pressure as low as possible during post injection intended for raising the exhaust temperature.
To accomplish the above object, the present invention provides an accumulator fuel injection system comprising: a first accumulator for storing high-pressure fuel pressurized by a pump; a fuel injection nozzle connected to said first accumulator via a fuel passage to inject fuel into a combustion chamber of an engine; a switching valve for connecting and disconnecting the high-pressure fuel stored in said first accumulator to and from said fuel passage; a second accumulator connected to a part downstream said switching valve in said fuel passage via a branch passage, said second accumulator storing fuel of lower pressure than the high-pressure fuel in said first accumulator; a pressure control valve provided in at least one of the part downstream said switching valve in said fuel passage and said second accumulator, said pressure control valve regulating a fuel pressure in said second accumulator and said fuel passage; an opening and closing valve for controlling injection of fuel from a fuel injection nozzle; main injection control device for controlling said switching valve and said opening and closing valve according to an operating state of the engine such that main fuel is injected from said fuel injection nozzle for a predetermined period of time; post injection control device for injecting additional fuel from said fuel injection nozzle after said main injection control device injects the main fuel, in a case where it is necessary to raise an exhaust temperature of the engine; and wherein said post injection control device provides control such that additional fuel is injected from said fuel injection nozzle when the fuel pressure in said fuel passage or said second accumulator has been reduced to a low pressure approximate to a pressure of the low-pressure fuel stored in said second accumulator via said pressure control valve.
Specifically, in a common rail system comprised of the first accumulator for storing high-pressure fuel and the second accumulator for storing low-pressure fuel of lower pressure than the pressure stored in the first accumulator, the additional fuel is injected from the fuel injection nozzle if the fuel pressure in said fuel passage or in the second accumulator has been reduced to a preset low pressure after said switching valve is switched for disconnection at a point in time when the injection of the main fuel by said main injection control device is finished, or after the injection of the main fuel by said main injection control device is finished.
The additional injection of the low-pressure fuel prevents the fuel from adhering to the wall surfaces of the cylinder liners while keeping the fuel pressure as low as possible during the post injection. The post injection control device injects the additional fuel after the main injection control device injects the main fuel for a predetermined period of time, and this causes the additionally injected fuel to be combusted by flame in the combustion chamber or reacted in the exhaust passage to raise the exhaust temperature. The additional injection by the post injection control device (post injection) is implemented by fuel that has been reduced to a low pressure approximate to the pressure of the low-pressure fuel stored in the second accumulator.
Therefore, the penetration power of the injected fuel is suppressed during the post injection to prevent the fuel from adhering to the surface walls of the cylinder liners. This enables activation of an after treatment device provided in an exhaust system, for example, by raising the exhaust temperature while preventing oil dilution, seizure, and the like.